1. Field of the Invention
This invention relates to cooling integrated circuits and more particularly to microchannel coolers for integrated circuits.
2. Description of the Related Art
Much of the power consumed by a modern integrated circuit (IC) during operation is dissipated as heat, increasing the temperature of the IC and altering its properties. For example, silicon switching speed is slower when hot. Additionally, reliability is reduced as temperature increases. If the temperature is sufficiently high, irreversible damage occurs. To remove heat as quickly as possible, several approaches have been used in the art. These generally involve mounting heat dissipaters, such as heat sinks, near the IC package.
FIG. 1 shows a cross-sectional view of one such prior art integrated circuit assembly 100. An IC 102 is mounted on a substrate 104 using C4 bumps 106. The package lid 108 sits on the substrate 104 and is thermally coupled to the IC 102 by a first level thermal interface material (TIM) 110. A heat exchanger 112, thermally coupled to the lid 108 by a second level TIM 114, removes heat to the ambient, which may be a gas, e.g., air or dry nitrogen, or a liquid, e.g., water or liquid nitrogen. The entire assembly 100 may be mounted to a circuit board (not shown) by pins 116 or any other convenient mounting mechanism.
As can be seen, this integrated circuit assembly relies on conduction through the thermal interface materials and the package to remove heat from ICs. Since many ICs typically have hotspots, i.e., they are not uniformly heated, that non-uniformity may be transferred largely unchanged through the package to the heat exchanger.
Referring to FIG. 2A, one approach contemplated in the prior art to provide improved cooling is to run a cooling liquid through channels 201 in the semiconductor as shown in FIG. 2A. A problem with the approach shown in FIG. 2A is that as the cool water enters one side 202 of the semiconductor and travels to the other, it loses cooling capacity by absorbing heat from the semiconductor as it travels. In that case, the other end 203 of the integrated circuit will be left with a higher temperature.
One approach to address that problem, shown in FIG. 2B, utilizes stacked channels 205 and 207 with cold water being supplied from two sides of an integrated circuit. As shown in FIG. 2B, cold water is supplied to the channels 205 on the bottom of front side 206 of the integrated circuit, and cold water is also supplied to the top channels 207 on the far side 208 of the integrated circuit. The cold water fed to channels 207 exits as warm water on side 206. Similarly, the cold water supplied on side 206 exits as warm water on far side 208. Thus, in order to deal with the problem associated with FIG. 2A (warm coolant concentrated on one side of the semiconductor), cold water is fed from both sides in FIG. 2B.
A vertical section close to the edge of side 208, and looking into the IC, is shown in FIG. 2C and a vertical section close to edge 206 is shown in FIG. 2D. A plan view of the integrated circuit shows the top channel 207 cold on side 208 and warm on side 206. Note that the transition shown as abrupt in FIG. 2E from cold to warm would, of course, be gradual.
The stacked channel approach shown in FIGS. 2B-2E suffers from the fact that the deep set of channels 205, closest to the heat source (i.e., the active semiconductor), shields the active semiconductor from the second set of channels 207. Further, each channel will have a “cold” end and then get warmer as the conduit crosses to the other side. Thus, some parts of the device are starved of coolant.
In view of the approaches described above, improved cooling approaches are desirable to improve reliability and performance.